In a known clamping device of this kind for axially clamping two machine parts disengageable from one another (German Patent Specification 38 14 550 C1), an axially movable actuating rod is provided for operating the clamping sleeve. As a rule, this actuating rod is a draw rod arranged in a machine tool spindle and which can be pushed away from the tool by a set of cup springs and be moved by means of a piston and cylinder unit in opposition to the force of the spring arrangement for releasing the clamping device. By shifting the clamping sleeve under the action of the set of cup springs, the clamping jaws are moved radially outwards and apply themselves, on the one hand, against the inclined side of a groove in the interior of the hollow centring extension or spigot and, on the other hand, against a conical surface of the tool receptacle. In this way, an axial gripping force is likewise produced. In consequence of the axially movable actuating rod and the set of cup springs acting on it, as well as of the piston and cylinder unit, this known clamping device has a rather great over-all length, however, and for this reason cannot be used in particular in turrets. In fact, a plurality of tool receptacles and tools is arranged on such turrets, so that very confined space conditions exist therein. Moreover, it is endeavoured to design turrets as short as possible in the axial direction.
In another known clamping device (German Patent Specification 27 36 412 C2), three locking balls which are radially movable in corresponding transverse bores in the centring extension are provided as clamping elements. A central pressure ball arranged in a central bore in the centring extension and supported against the locking balls serves as a clamping arrangement and can be pushed towards the tool by means of differently designed clamping facilities. In this way, the locking balls are thrust radially outwards and in the process press on the conical surface of the tool receptacle. This produces a pressing of the abutment surface of the tool into contact against the end face. Owing to the centrally arranged pressure ball and the actuating arrangement acting on it likewise in the region of the central axis, it is not possible to employ this clamping device for so-called driven tools. In fact, such driven tools require, both in the centring extension and in the tool receptacle, a central free space for passing through a driving shaft or driving rod with which the tool can be driven rotationally and also adjusted. The locking balls and the pressure ball have the disadvantage that clamping forces are transmitted in point form only in three places. This results in a high specific surface pressure and, consequently, also relatively great wear not only at the balls themselves, but also at the conical surface cooperating with the locking balls on the tool receptacle. Also, on account of the high specific surface pressure, the maximum clamping force and, consequently, the force with which abutment surface and end surface are pressed against one another is small. Furthermore, the known clamping device also has the disadvantage that an important part of the device, namely the balls, is arranged in the tool itself. Each individual tool must therefore be equipped with such balls and corresponding bores and holding arrangements for the balls, which increases the cost of manufacturing the tools.